Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a wide variety of signal transduction processes within the cell (Hardie, G. and Hanks, S. (1995) The Protein Kinase Facts Book. I and II, Academic Press, San Diego, Calif.). The kinases may be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Sequence motifs have been identified that generally correspond to each of these kinase families (e.g., Hanks, S. K., Hunter, T., FASEB J., 9:576-596 (1995); Knighton, et al., Science, 253:407-414 (1991); Hiles, et al., Cell, 70:419-429 (1992); Kunz, et al., Cell, 73:585-596 (1993); Garcia-Bustos, et al., EMBO J., 13:2352-2361 (1994)).
Protein kinases may be characterized by their regulation mechanisms. These mechanisms include, for example, autophosphorylation, transphosphorylation by other kinases, protein-protein interactions, protein-lipid interactions, and protein-polynucleotide interactions. An individual protein kinase may be regulated by more than one mechanism.
Kinases regulate many different cell processes including, but not limited to, proliferation, differentiation, apoptosis, motility, transcription, translation and other signalling processes, by adding phosphate groups to target proteins. These phosphorylation events act as molecular on/off switches that can modulate or regulate the target protein biological function. Phosphorylation of target proteins occurs in response to a variety of extracellular signals (hormones, neurotransmitters, growth and differentiation factors, etc.), cell cycle events, environmental or nutritional stresses, etc. The appropriate protein kinase functions in signalling pathways to activate or inactivate (either directly or indirectly), for example, a metabolic enzyme, regulatory protein, receptor, cytoskeletal protein, ion channel or pump, or transcription factor. Uncontrolled signalling due to defective control of protein phosphorylation has been implicated in a number of diseases, including, for example, inflammation, cancer, allergy/asthma, disease and conditions of the immune system, disease and conditions of the central nervous system, and angiogenesis.
Aurora A Kinase
The Aurora kinases are a family of serine/threonine protein kinases critical for the proper regulation of mitosis. Of the three mammalian aurora paralogues, A, B and C, Aurora A and B are commonly overexpressed in human tumours. Aurora A is amplified in many human tumours and is involved in centrosome duplication, bipolar mitotic spindle formation, chromosome alignment and spindle checkpoint.
Yes Kinases
Yes is a member of the Src family of non-receptor tyrosine kinases. Like Src, Yes amplifies signals generated by various receptors, in many cases duplicating the functionality of Src. Expression of Yes is elevated in melanocytes and in melanoma cells, and Yes kinase activity is stimulated by neurotrophins, which are mitogenic and metastatic factors for melanoma cells. In addition to melanoma, yes is also over-expressed in colon cancer.
Abl Kinases
Abl is a non-receptor tyrosine kinase distinguished by its localization in the nucleus as well as the cytoplasm. Cytoplasmic Abl associates with F-actin and is capable of stimulating cell growth. Nuclear Abl is thought to participate with DNA-PK and ATM to initiate signalling in response to DNA damage. Chromosomal translocations involving Abl and the breakpoint cluster region on chromosome 22 produce the bcr-Abl fusion protein, resulting in a constitutively active Abl thought to be critical in the pathogenesis of chronic myelogenous leukemia (CML).
cSRC Kinases
c-Src is a non-receptor tryrosine kinase belonging to a family including c-Src, c-Yes and Fyn. It is expressed in most cell types at low levels and is activated by dephosphorylation of the Tyr530 site and phosphorylation of a second site, Tyr416, present within the kinase domain. The c-Src oncogene has been implicated in the development of growth, progression and metastasis in a variety of human cancers.
Vascular Endothelial Growth Factor (VEGF)
Vascular endothelial growth factor (VEGF) has a pivotal role in stimulating the pathological angiogenesis required for a number of diseases including cancer. Its expression has been shown to be up-regulated in a variety of tumours and has been reported to be a prognostic indicator of tumour progression and/or decreased survival. VEGF induces a signalling response by binding to VEGF receptor-1 (Flt-1), VEGF receptor-2 (KDR/Flk-1) and VEGF-3 (Flt4).
Activation of the VEGF receptor-2 is considered to be the prominent mechanism by which VEGF induces endothelial cell proliferation and migration.
The functions of VEGF-C and VEGF-D and their receptor Flt4 have been implicated in the onset and promotion of tumour cell metastasis in colorectal cancer (Hebei Yixue (2007), 13(10), 1135-1140); in the metastasis of primary breast cancer (Dalian Yike Daxue Xuebao (2005), 27(6), 413-417) and in lymphatic metastasis in patients with lung cancer (Zhongguo Yike Daxue Xuebao (2005), 34(3), 244-245).
FLT3 Kinase
The ligand for the FMS-like tyrosine kinase 3 (FLT3) receptor is an early acting growth factor in tumour growth events and supports survival, proliferation and differentiation of primitive hemopoietic progenitor cells. Signalling through the FLT3 receptor activates several downstream signalling pathways such as RAS/Raf/MAPK and PI3 kinase cascades (see Takahashi, S. et. al.; Leukemia Research, 29(8), 893-899, 2005. Namikawa, R. et. al.; Stem Cells, 14, 388-395, 1996).
Mutations in the FLT3 gene are amongst the most frequent genetic defects found in acute myeloid leukemia (AML) (see Kottaridis, P. D.; Gale, R. E.; Langabeer, S. E.; Frew, M. E.; Bowen, D. T., Linch; D. C.; Blood, 2002).
Most mutations lead to a constitutively activated receptor which gives rise to an oncogenic nature. Major mutations are length mutations (FLT3-LM also called FLT3-ITD) and tyrosine kinase domain mutations (FLT3-TKD) (see Kiyoi, H.; Chno, R. et. al.; Oncogene, 21(16) 2555-2565, 2002. Yamamoto, Y.; Kiyoi, H.; Blood, 97(8), 2434, 2439, 2001). The latter has been suggested to trigger both activation loop and stabilisation of the active site.
Inhibitors of FLT3 are therefore expected to be of benefit to patients suffering from AML and to inhibit the early angiogenesis events associated with tumour growth and metastasis.
FLT4 Kinase
FMS-like tyrosine kinase 4 (FLT4) is a Vascular endothelial growth factor (VEGF) receptor which is activated by both VEGF-C & VEGF-D. Both of these growth factors have been shown to induce lymphangiogenesis via FLT-4 (see Schneider, M.; Buchler, P.; Giese, N.; Wilting, J.; Buchler, M. W.; Friess, H.; Int. J. Oncol., 2006, 28: 883-890) and have also both been shown to be lymphangiogenic in tumours, stimulating metastasis.
Recent research (see Alitalo, K. et. al.; Cancer Cell, 2008 13, 554-556 and Alitalo, K. et. al.; Nature, 2008 454, 656-660) has shown that targeting inhibitors towards the FLT-4 receptor may provide additional efficacy for anti-angiogenic therapies, especially towards vessels that are resistant to VEGF or VEGFR-2 inhibitors.
Trk Kinases
The Trk family of receptor tyrosine kinases include Trk A, Trk B, and Trk C. Trk A is the high-affinity receptor for NGF, while Trk B is the high-affinity BDNF receptor, and Trk C serves as a receptor for neurotrophin-3 (NT-3). The signalling activity of these receptors includes the Ras, MAP kinase pathway, and the PI3-kinase pathway. It is envisaged that inhibitors of Trk kinases such as Trk A will be useful in cancer therapy.
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